CD40, also known as bp50 or CDW40, is a type I membrane protein found on the surface of B cells and primary carcinomas. It belongs to the TNF-R family. CD40 serves as the receptor for CD40 ligand (CD40L), a cytokine expressed on the cell surface of T cells. CD40 plays a critical role in B cell proliferation, antibody class switching, modulation of apoptosis in the germinal center through interaction with T cells expressing CD40L, and activation of CD4+ T cells. Ligation of CD40 with its ligand preferentially induces stress-activated protein kinases in B lymphocytes.